Let’s deprecate UPDATE FROM!

I guess that many people using UPDATE … FROM on a daily basis do so without being aware that they are violating all SQL standards. All versions of the ANSI SQL standard that I checked agree that an UPDATE statement has three clauses – the UPDATE clause, naming the table to be updated; the SET clause, specifying the columns to change and their new values; and the optional WHERE clause to filter the rows to be updated. No FROM or JOIN – if you need data from a different table, use a subquery in the SET clause. The optional FROM…
Read More

Want a Service Pack? Ask for it!

32 Comments
Service pack 2 for SQL Server 2005 is already 11 months old. And there is still no sign of service pack 3 on the horizon. Why is that? Has Microsoft managed to release a perfect, completely bug-free product? No, of course not – with the size and complexity of a product such as SQL Server is, that will simply never happen. There have, in fact, enormous numbers of bugs been uncovered and fixed since SP2 was released. And roughly once every two months, a so-called “cumulative update package” gets released. The last one is officially called “Cumulative update package 5…
Read More

Bin packing part 3: Need for speed

2 Comments
In the first post of this series, I explained the bin-packing problem and established a baseline solution. The second post investigated ways to increase the packing efficiency. In none of these posts did I pay particular attention to performance – and frankly, it shows. Performance of all solutions presented thus far sucks eggs. Time to see what can be done about that. If you look in detail at the most efficient solution so far (dbo.OrderDesc, as described in the second post of the series), you’ll see that all 40,000 registrations are processed one by one, with a cursor. No surprise…
Read More

Bin packing part 2: Packing it tighter

No Comments
In my previous post, I explained the bin packing problem, explained an example scenario, and established a baseline for both speed and efficiency of bin packing algorithms by writing a rather crude cursor-based procedure. In this part, I will look at some small modifications that can be made to this code to make it better at packing bins as full as possible, so that less bins will be needed. Reordering the cursor The Baseline procedure didn’t process the registrations in any particular order. Granted, there is an ORDER BY Year, Quarter in the cursor declaration, but that is only needed…
Read More

Bin packing part 1: Setting a baseline

No Comments
Some problems can only be solved by brute-forcing every possible combination. The problem with such an approach, is that execution time grows exponentially as the amount of input data grows – so that even on the best possible hardware, you will get inacceptable performance once the input data goes beyond the size of a small test set. These problems are called “NP-complete” or “NP-hard”, and the most viable way to deal with them is to use an algorithm that finds a solution that, while not perfect, is at least good enough – with a performance that, while not perfect, is…
Read More

Poor men see sharp – more cursor optimization

5 Comments
After making my post on cursor optimization I received some comments that triggered me to do some further investigation. Adam Machanic wrote in my blog’s comments that using SQLCLR to loop over a SqlDataReader would be much faster than any T-SQL based cursor. And Erland Sommarskog wrote in a usenet thread that he has some colleagues who think that a “poor man’s cursor” is always better than a real cursor. So I decided to give these options a try and see what comes out in a real test. I simply reused the test cases I had already used for testing…
Read More

Curious cursor optimization options

13 Comments
The best way to optimize performance of a cursor is, of course, to rip it out and replace it with set-based logic. But there is still a small category of problems where a cursor will outperform a set-based solution. The introduction of ranking functions in SQL Server 2005 has taken a large chunk out of that category – but some remain. For those problems, it makes sense to investigate the performance effects of the various cursor options. I am currently preparing a series of blog posts on a neat set-based solution I found for a problem that screams “cursor” from…
Read More

So-called “exact” numerics are not at all exact!

15 Comments
Attempting to dispel myths tends to make me feel like Don Quixote, riding against hordes of windmills that won’t budge. In this case, even some of my fellow MVPs and Microsoft’s own Books Online are among the windmills… Books Online says that there are two categories of numeric data types: “approximate” (float and real), and “exact” (all others, but for this discussion mainly decimal and numeric). It also says that “floating point data is approximate; therefore, not all values in the data type range can be represented exactly”, thereby suggesting that other numeric data types are capable of representing all…
Read More

How NOT to pass a lot of parameters

Did you know that SQL Server allows stored procedures to have up to 2100 parameters? And more important: do you care? Well, some people do care, and Joe Celko seems to be one of them. If you are a regular reader of SQL Server newsgroups, you probably know Joe Celko from his always unfriendly and often incorrect replies. Here is a typical example, one that I have seen several times recently, in a paraphrased form: Question: I want to send a list of values to my stored procedure, but WHERE ColumnName IN (@ValueList) does not work – how to solve…
Read More

What if null if null is null null null is null?

16 Comments
In this fourth and final part in my series about NULL, I’ll discuss some well-known and some less well-known functions and keywords that are specifically created to deal with NULL values. And I will, of course, explain why null if null is null null null is null. In case you have not yet read them, you can click these links to read the first, second, and third part.   IS NULL is not = NULL   I have already explained why tests for equality with NULL will always return Unknown instead of True or False. This holds true in all…
Read More
Menu

By continuing to use the site, you agree to the use of cookies. more information

The cookie settings on this website are set to "allow cookies" to give you the best browsing experience possible. If you continue to use this website without changing your cookie settings or you click "Accept" below then you are consenting to this.

Close